During evaporation and/or condensation, fluid in a heat transfer system is typically present in two phases, i.e. liquid and gas phase. The amount of fluid that is in the liquid phase and the amount of fluid that is in the gas phase can for example be expressed by the void fraction. The void fraction for a section of a tube in which the two-phase system exists is the ratio of the surface in the cross section that is in the gas phase to the total surface in the cross-section for the tube. For reasons of clarity, this is referred to as the cross-sectional void fraction. It is used as one of the key parameters for characterizing a two phase system.
Quite a number of experimental methods have already been proposed to measure the void fraction of a two phase system. Yet, most of these methods are either intrusive (disturb the liquid-gas flow), such as the conductive methods, not widely applicable (the optical methods) or very complex and expensive (e.g. gamma or neutron attenuation). Moreover, due to the physical limitations of many sensors mostly a volumetric (volume averaged) void fraction is determined.
In optical methods, visual images of the flow are processed to determine the void fraction. This requires a transparent tube (glass or plastic) which limits the possible temperatures and pressures. In International Journal of Heat and Mass Transfer, 48 (2005) 2970, Thome et al. developed an optical technique to determine the void fraction which showed a good agreement with the Rouhani-Axelsson drift flux void fraction model, described in Journal of Heat and Mass Transfer 13 (1970) 383. However, this optical method could only be used for stratified flow regimes, which limits the applicability of the measurement technique and can only be used when applying transparent tubes. Methods based on X-ray attenuation, do not require a transparent tube, yet there are serious cost and safety issues connected to this method.
Ultrasonic transmission techniques detect changes in acoustic impedance which is closely related to the density of the media. However, a gas-liquid interface acts almost as a perfect mirror for an acoustic wave. This technique can therefore only be used for total void fractions up to about 20%, which does not cover the full range of interest.
Capacitive void fraction measurements are often used, because they are quite easy to implement, non-intrusive and relatively low cost compared to some other techniques. A typical example for using capacitive measurements is based on measuring the volume averaged capacity between two curved electrodes mounted on the tube wall, the electrodes forming a capacitor. Nevertheless, due to the curvature of the electrodes, the measured capacitance is not only dependent on the void fraction but also on the spatial distribution of the phases. Hence, the measured capacitance does not vary linearly with the void fraction. In International Journal of Heat and Mass Transfer 53(2010) 5298, Caniére et al. describes an example of the use of a capacitance sensor for characterizing a flow regime of a refrigerant based on the temporal and relative magnitude evolution of the capacitance of the capacitor. Nevertheless, the capacitive sensor used provides a signal related to the capacitance, but not to the actual void fraction value. Consequently, such methods do not allow void fraction measurement. In Flow Measurement and Instrumentation 10 (1999) 65, Keska et al. made a comparison of four techniques to measure the flow behaviour: a resistive method, a capacitive method, an optical method and a static pressure based method. It was concluded that the capacitive and resistive methods were both very effective to measure the flow behaviour.
In Journal of the Korean Nuclear Society 17 (1985) 1, Moon-Hyun et al. indicated that the relationship between the measured capacitance and the void fraction depends on the occurring flow regime. Moon-Hyun et al. disclosed steady (state) or stationary experiments in which the flow regime is known a priori because it is controlled in the experimental set-up. Dynamic or unsteady experiments were not discussed.
Therefore, there is still a need for an improved method and device for determining the void fraction of a multi-phase system, the method and device being usable in both steady as well as unsteady flow regimes.
An example of a system wherein a multi-phase system occurs is a refrigerator. In a refrigerator, the gas phase of a refrigerant typically has a much lower density than the liquid phase. During evaporation, the flow will accelerate due to this density change, resulting in the occurrence of high velocities in the flow. Such high velocities cause a lot of friction and therefore pressure drop. To limit this pressure drop, the refrigerant flow is typically divided in several parallel circuits. At the inlet of the evaporator, a distributor with capillary tubes divides the refrigerant over the several circuits. At the outlet, the different circuits are connected to a collector, from which the refrigerant flows through a single tube again.
It is absolutely essential that the refrigerant at the outlet of the evaporator is fully evaporated and no liquid droplets remain in the flow. If liquid droplets would still be present, this would damage the compressor. Nevertheless, the heat transfer in each circuit can differ due to the variation of the position and amount of tubes. Typically, at the distributor, there is already two phase flow present. This two phase flow is distributed over the different parallel circuits in such a way that the refrigerant exits each circuit in the same slightly overheated state.
Due to the difference in heat flux for each circuit, attaining the same slightly overheated state for each circuit can only be obtained by proper design of the heat transfer system. Typically, in order to obtain a proper design, the capillary tubes of the different circuits are selected to have different lengths to obtain the proper conditions. Mostly, a trial and error method is used to determine the length of each capillary tube. If the tube is too short, the mass flow rate of refrigerant in the circuit will be to high and the refrigerant will not be fully evaporated at the outlet. If the tube is too long, the mass flow rate will be to small and the refrigerant will be more overheated than necessary. The above method for designing is labour intensive and sub-optimal.